Synthetic membranes are used for a variety of applications including desalination, gas separation, filtration and dialysis. The properties of the membranes vary depending on the morphology of the membrane ie properties such as symmetry, pore shape and pore size and the polymeric material used to form the membrane.
Different membranes can be used for specific separation processes, including microfiltration, ultrafiltration and reverse osmosis. Microfiltration and ultrafiltration are pressure driven processes and are distinguished by the size of the particle or molecule that the membrane is capable of retaining or passing. Microfiltration can remove very fine colloidal particles in the micrometer and submicrometer range. As a general rule, microfiltration can filter particles down to 0.1 μm, whereas ultrafiltration can pass through particles as small as 0.01 μm. Reverse Osmosis operates on an even smaller scale.
As the size of the particles to be separated increases so too does the pressure required to carry out the separation and the density of the membrane.
A large surface area is needed when a large flux is required. One known technique to make filtration apparatus more compact is to form membranes in the shape of a hollow porous fiber. Modules of such fibres can be made with an extremely large surface area per unit of volume.
Microporous synthetic membranes are particularly suitable for use in hollow fibres and they are produced by phase inversion. In this process, a polymer is dissolved in an appropriate solvent and a suitable viscosity of the solution is achieved. The polymer solution can then be cast as a film or hollow fiber, and then immersed in a precipitation bath such as water. This causes separation of the homogeneous polymer solution into a solid polymer and liquid solvent phase. The precipitated polymer forms a porous structure containing a network of uniform pores. Production parameters that affect the membrane structure and properties include the polymer concentration, the precipitation media and temperature and the amount of solvent and non-solvent in the polymer solution. These factors can be varied to produce microporous membranes with a large range of pore sizes (from less than 0.1 to 20 μm), and altering chemical, thermal and mechanical properties.
Microporous phase inversion membranes are particularly well suited to the application of removal of viruses and bacteria. Of all types of membranes, the hollow fiber contains the largest membrane area per unit volume.
Different techniques can be used to induce phase separation and prepare polymer membranes. A polymer dissolved in a solvent can solidify upon cooling, which is known as liquid-solid phase separation. Phase separation can be induced either by a temperature change or by a change in the concentration of the solution. These two processes are referred to as thermally induced phase separation (TIPS) and diffusion induced phase separation (DIPS). The morphology induced by phase separation needs to be secured and hence solidification of the polymer phase needs to be achieved. In the TIPS process this is usually done by dropping the temperature below the g transition temperature or the melting point of the polymer. The DIPS process uses a change in concentration, caused by diffusion of a solvent and a non-solvent, to induce a phase separation. With this technique, the hollow fiber membranes are produced using a batchwise process. The DIPS process has an advantage that asymmetric membranes can easily be formed. In addition, the spinning of hollow fibers can be performed at room temperature, whereas the alternative process—thermally induced phase separation (TIPS) requires much higher temperatures. Since DIPS uses the diffusion of non-solvent and solvent it is relatively easy to control the rate at which membrane formation takes place by changing the concentration of the non-solvent bath and the polymer solution. The disadvantage however, is that macrovoids can be produced, in the form of fingerlike intrusions in the membrane. They decrease the mechanical strength of the membrane but can be avoided by choosing the right composition of solution.
Flat sheet membranes are prepared in the following way. A polymer solution consisting of a polymer and solvent is brought into contact with a non-solvent. The solvent diffuses outwards into the coagulation bath and the non-solvent will diffuse into the cast film. After a given period of time, the exchange of the non-solvent and solvent has proceeded such that the solution becomes thermodynamically unstable and demixing occurs. Finally a flat sheet is obtained with an asymmetric or symmetric structure
Hydrophobic surfaces are defined as “water hating” and hydrophilic surfaces as “water loving”. Many of the polymers that porous membranes are made of are hydrophobic. Water can be forced through a hydrophobic membrane by use of sufficient pressure, but the pressure needed is very high (150-300 psi), and the membrane may be damaged at such pressures and generally does not become wetted evenly.
Hydrophobic microporous membranes are characterised by their excellent chemical resistance, biocompatibility, mechanical strength and separation performance. Thus, in the application of water filtration, such hydrophobic membranes need to be hydrophilised to allow water to permeate them. Many hydrophilic materials are not suitable for MF and UF membranes that require mechanical strength and thermal stability since water molecules play the role of plasticizers.
Currently, poly(tetrafluoroethylene) Wipe), Polyethylene (PE), Polypropylene (PP) and polyvinylidene fluoride (PVDF) are the most popular and available hydrophobic membrane materials. Polyvinylidene fluoride (PVDF) is a semi-crystalline polymer containing a crystalline phase and an amorphous phase. The crystalline phase provides good thermal stability whilst the amorphous phase has flexibility towards membranes. PVDF, exhibits a number of desirable characteristics for membrane applications, including thermal resistance, chemical resistance (to a range of corrosive chemicals, including chlorine), and weather (UV) resistance
Modification of a polymer's surface potentially can maintain a polymer's desirable bulk properties but can provide new, different interfacial properties. Membranes made from hydrophilic polymers are generally less prone to fouling thane hydrophobic polymers. In some instances, surface modification of the more chemically resistant polymers has rendered them less susceptible to fouling. Numerous techniques exist for the surface modification of polymers. The most common examples of this chemistry are reactions that introduce a single type of functional group or mixture of functional groups.
In general, all techniques of hydrophilisation of polymer surfaces involve an increase of the surface amount of polar groups. From a microscopic point of view, the basis of surface hydrophilisation is to maximise hydration and hydrogen bonding interactions. All sorts of oxygen, nitrogen or sulfur containing organic functional groups can interact with water more effectively than common carbon based repeating units. There are various methods for wetting a membrane on a non-permanent basis. One method of hydrophilising a porous hydrophobic membrane has been to pass alcohol through the pores of the membrane, then replace the alcohol by water. Surfactants and a post treatment with a glycerol coating have also been used. This is an adequate solution to the problem, so long as the water remains in the pores. However, if the water is removed from the pores either wholly or partially, and they are filled with air, the hydrophilised membrane is rendered hydrophobic again, and water cannot pass through the pores if it is not subjected to high pressure.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.